FIG. 6 is a cross-sectional view showing a construction of a prior art Schottky barrier diode photodetector, which has a similar construction as that reported in "Physics of Semiconductor Devices", by S. M. Sze, vol. 2 p. 289.
In FIG. 6, reference numeral 1 designates an n type silicon substrate. A metal film 4 is produced on the n type silicon substrate 1. A constant voltage power supply 5 is provided for biasing a Schottky barrier diode comprising the silicon substrate 1 and the metal film 4. A resistor 6 is provided for converting a signal current into a voltage-signal. Reference numeral 7 designates light incident on the photodetector.
FIG. 7 is an energy-band diagram depicted along the broken line b--b' of FIG. 6. In FIG. 7, reference numeral 8 designates an energy level within the band gap due to impurities or crystal defects in the n type silicon substrate. A photo-electron 9 is excited by the incident light 7. An electron 10 is thermally excited via the energy level 8. A hole 11 is generated by thermal excitation. Reference character I.sub.L designates photoelectric current. Dark current I.sub.G is generated by thermal excitation.
The device will be operated as follows.
A photo-electron 9 excited by the incident light 7 in the metal film 4 has such a large kinetic energy that it enters the n type silicon passing over a Schottky barrier produced between the silicon substrate 1 and the metal film 4. All the photo-electrons which have entered the n type silicon are accelerated by the electric field which exists in the depletion layer and are collected at the side of the rear surface electrode on the n type silicon substrate 1. This photoelectric current I.sub.L is converted into a voltage by a resistor 6, thereby enabling an optical signal 7 to be extracted in the form of voltage signal V.sub.L.
On the other hand, in a depletion layer which has spread into the n type silicon substrate relative to the Schottky junction, an electron 10 which is easily thermally excited through the energy level 8 as a stepping stone is attracted by the electric field in the depletion layer similarly as the case of photo-electron 9, and every electron 10 is collected at the side of the n type silicon substrate 1. At this time, a hole 11 is also collected at the side of the metal film 4 by means of the electric field.
In this way, a total current I at the Schottky junction is equivalent to a value which is obtained by adding the dark current I.sub.G to the photoelectric current I.sub.L, that is, EQU I=I.sub.L +I.sub.G ( 1)
The dark current I.sub.G is converted into a voltage by the resistor 6 similarly as the photoelectric current I.sub.L and therefore the signal voltage V.sub.L is as follows: EQU V.sub.L =RI=R(I.sub.L +I.sub.G)=RI.sub.L +RI.sub.G ( 2)
This means that the original optical signal component RI.sub.L is combined with the dark current component RI.sub.G.
A more complex circuit is required for extracting only the optical signal component RI.sub.L by removing the dark current component RI.sub.G. For example, in a case where an imaging device or the like is constituted by arranging semiconductor photodetectors 15 having the above-described structure in an array configuration on a plane 16 as shown in FIG. 8 to obtain a two-dimensional image, variations in the crystallization and in the amount of impurities in the silicon substrate of respective photodetectors 15 cause differences in the dark currents of these photodetectors, thereby resulting in noise in the two-dimensional image.
Furthermore, since the dark current is generated only in the depletion layer, it increases with increased reverse bias voltage, that is, with increased depletion layer width. The dark current also increases with increased temperature.
In a case where a p type silicon substrate is used in place of n type silicon substrate, the same effects are observed when a positive voltage is applied to the metal film 4 so that a hole functions in place of an electron as described.
The prior art semiconductor photodetector constituted as described above has a problem in that the dark current operates as noise in the detected signal.